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Discussions are taking place on how to use the US military to deter and contain Iranian activities

Iran's apparent direct military support for rebels in Yemen is an urgent concern

(CNN)Even as the Pentagon remains focused on deterring North Korea and fighting ISIS, multiple US officials tell CNN that Defense Secretary James Mattis is asking planners to think about what action to take on Iran.

Discussions are taking place on how to use the US military to deter and contain Iranian activities in neighboring countries, short of a direct conflict. The Pentagon has not commented publicly, but one official familiar with the Mattis's thinking said, "A lot of thought is being put into Iran."

This comes as the Trump administration is trying to assemble the equivalent of an international diplomatic and economic coalition in practice, if not in name, to counter what it sees as Iran's growing influence across the Middle East. Secretary of State Rex Tillerson this week convened a meeting between the US, UK, France and Germany to coordinate US-European cooperation to counter Iran through diplomatic and economic means.

Nobody in the Trump administration is talking about starting a war even in the wake of the tough talk about the Iran nuclear agreement. But there is a consensus about the threat of what US officials call Iran's "malign influence" across the region.

"What the Iranians have done across the broader Middle East is fuel and accelerate these cycles of violence so that they can take advantage of these chaotic environments, take advantage of weak states, to make them dependent on them for support," the National Security Advisor HR McMaster told a security forum last weekend.

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We have to address what is a growing Iranian capability and an ability to use proxies, militias, terrorist organizations to advance their aim, their hegemonic aims in the region," McMaster added.

Yemen is becoming a focal point for US-Iranian tensions. The US has been supporting a longtime Saudi air campaign targeting Iranian backed Houthi rebels in Yemen. Thousands of civilians have been killed and the country has been hit by a cholera outbreak and famine.

The dire situation in Yemen has sparked outrage around the world. So if the US wants to deter Iran's influence in the country, it would need to send a strong signal to allies that the Saudis have to stop airstrikes that are killing civilians and provide aid.

It may not be directly related but on Wednesday President Donald Trump made an unusual humanitarian gesture toward Yemen calling for Saudi Arabia to end its blockade of the country's ports.

Iran's apparent direct military support for rebels in Yemen is an urgent concern. US intelligence analysts have now looked at the remnants of a missile the Houthis fired last month that landed close to the commercial airport in the Saudi Arabian capital Riyadh. Those remnants indicate the missile was provided by Iran, and the assessment is the launcher and missile training was also provided by Tehran as part of its effort to challenge the Saudi regime.

US officials also believe the Iranians continue to ship weapons into Yemen over land smuggling routes through Oman or by using small boats to sail to Yemeni ports.

Iran's activities throughout the region are infuriating key Administration officials. It's no secret that Mattis has long viewed Iran as a major threat in the Middle East. And now CIA director Mike Pompeo -- who reports suggest may be the next Secretary of State -- has fired his own written shot across the bow.

Pompeo says he wrote a note to Qasem Soleimani, head of Iran's Quds Force: "I sent it because he had indicated that forces under his control might in fact threaten US interests in Iraq."

Pompeo says Soleimani refused to open the letter but the message was still clear. "What we were communicating to him in that letter is that we will hold he and Iran accountable for any attacks on American interests in Iraq by forces that are under their control. And we wanted to make sure that he and the leadership in Iran understood that in a way that was crystal clear."

While diplomatic and economic efforts such as sanctions will continue, it's not clear how, or if, the Pentagon can develop an effective deterrence or containment strategy. Putting more ships, including aircraft carriers, or other assets such as drones into the region as a show of force and efforts to collect more military intelligence on Iran, would be typical moves officials say. But commanders are aware many assets are assigned to current high priorities such as the Korean Peninsula and the fight against ISIS in Syria and Iraq.

The challenge is made stiffer by the fact the Pentagon is facing an Iranian challenge on multiple fronts. McMaster pointed out that in Iraq and Syria alone: "What they are in the middle of doing now is using a sophisticated campaign of subversion in Iraq and a continued support for Assad. About 80% of Assad's fighters are Iranian proxies in Syria to establish this kind of land bridge over into the Mediterranean and so what we face is Iran having a proxy army on borders of Israel."

McMaster added that "what we see is the weaponization of Iran's network in Yemen, in southern Lebanon, they are trying to do it in Syria with these long-range missiles and they are increasing the threat that Iran poses to the whole region," says McMaster.

For now, there are no indications the Pentagon has launched any new initiatives on Iran but the county's neighbors are increasingly worried. US officials say Jordan is concerned that Iranian backed fighters on Syria's southwest border could move close to the Jordanian border.

And Israel may not be waiting. In the last several days they have launched airstrikes against suspected Iranian sites inside Syria, according to US officials.